This invention relates to the preparation of polycarbonates, and more particularly to their preparation by a method requiring enhancement of crystallinity.
Solid state polymerization (hereinafter sometimes "SSP") of polycarbonates is disclosed, for example, in U.S. Pat. Nos. 4,948,871, 5,204,377 and 5,214,073, the disclosures of which are incorporated by reference herein. It involves a first step of forming a precursor polycarbonate, typically a prepolymer formed by melt polymerization (i.e., transesterification) of a dihydroxyaromatic compound such as bisphenol A with a diaryl carbonate such as diphenyl carbonate; a second step of enhancing the crystallinity of the prepolymer; and a third step of building the molecular weight of the crystallized prepolymer by heating to a temperature between its glass transition temperature and its melting temperature. Use of this polymerization method is of increasing interest by reason of its effectiveness and environmental benefits.
The second or crystallinity enhancement step of this method is, at least in part, required to suppress fusion of the particles of precursor polycarbonate during the SSP step. An essential chemical feature of the SSP step is removal of hydroxyaromatic compound, usually phenol, from the polymer end groups, permitting molecular weight increase. Such removal must occur uniformly throughout the mass of precursor polycarbonate, which cannot happen if the particles fuse into larger agglomerates. By crystallinity, throughout the mass or at least on the surfaces of the particles thereof, fusion is inhibited.
Crystallinity enhancement is performed, according to the prior art, by solvent treatment or heat treatment. As described, the solvent treatment method may in fact employ a good or poor solvent for the prepolymer, with contact involving either the liquid or vapor form thereof. Illustrative "solvents" include aliphatic aromatic hydrocarbons, alcohols, ethers, esters, ketones, halogenated aliphatic and aromatic hydrocarbons, and diaryl carbonates.
The use of "solvents" for crystallinity enhancement requires a separate step of contact of the precursor polycarbonate with the "solvent". Only after that separate step is completed can SSP be undertaken. For economy of operation, it would be preferred to perform crystallinity enhancement and SSP in a single, continuous sequence not requiring steps such as removal of an extraneous liquid. Enhancement by heat treatment, i.e., thermally, would be preferable for this purpose.
As disclosed in the aforementioned U.S. Pat. No. 4,948,871, however, thermal crystallinity enhancement requires a temperature between the glass transition temperature (Tg) and the melting temperature of the precursor polycarbonate. Any heating above the Tg has the effect of causing fusion of the precursor particles into larger agglomerates, which is counterproductive for phenol removal. Thus, the very operation that makes SSP possible also inhibits it.
It would be desirable, therefore, to develop a thermal crystallinity enhancement method which can, when desired, be combined with pelletization without forming large agglomerates from the pellets or other particles of precursor polycarbonate. It would be particularly desirable to develop a method which can be integrated with SSP.